FR2629562A1 - METHOD AND DEVICE FOR COATING WELDED TUBES - Google Patents

Abstract

The process is intended for coating metal tubes comprising a weld bead 2 in which several layers of thermoplastic material are applied to the tube 1 by means of a strip 21 wound around the tube. After the application of a first layer of thermoplastic material, one applies, straddling the weld bead 2, sheets F1, F2, F3, F4 of thermoplastic material which are juxtaposed one next to the other with a certain edge-to-edge overlap, so that their juxtaposition covers the weld bead 2 over its entire surface as well as a narrow area surrounding it on both sides, then the (or ) next layer (s) of thermoplastic material, which (which) covers these sheets F1, F2, F3, F4. Use in particular to avoid any reduction in thickness of the thermoplastic material coating in line with the weld bead of the metal tubes.

Description

The present invention relates to a method and a device for

  coating, by protective layers of thermoplastic material, welded metal tubes, in particular steel, and having a weld bead in relief on their outer surface. To protect the metal tubes, especially the steel tubes, against corrosion, they are covered with a coating which is sometimes bitumen, but often a thermoplastic synthetic material, such as polyethylene,

polypropylene or the like.

  The method generally consists in applying, on the surface of the preheated tube, first a liquid or viscous glue or powder (especially epoxy glue), and / or a thin film of adhesive called "primary", then the thermoplastic material in the form of a thin strip obtained by hot extrusion through a flat die. This strip of thermoplastic material winds helically around the tube which simultaneously rotates about its axis and advances longitudinally along this axis. In this way, a layer of thermoplastic material is obtained in several layers.

  homogeneous, usually a few millimeters thick.

  This method is easily applied to tubes without

  solder, whose surface is regular.

  But with welded tubes, there are difficulties.

  These tubes, especially those of large diameter, have indeed an apparent and prominent weld seam (as shown in Figure 1). This cord may be either longitudinal, that is to say along a generatrix of the tube, as shown in Figure 2, or helically as shown

in Figure 3.

  When coated, as explained above, a welded tube by maintaining constant its rotation speed and the tension of the thermoplastic material strip, the creep of the heat-set thermoplastic material makes that: the thickness of the deposited coating on the weld bead is less than the thickness deposited on the rest of the circumference of the tube (see Figure 4), and, in addition: - it can appear, on both sides of the weld bead, empty spaces between the surface of the tube and the plastic strip which, stretched between the extruder and the tube, is a bridge between the top of the weld and the

tube surface (see Figure 5).

  These effects are harmful to the protection of the tube.

  For effective protection requires a minimum thickness imposed by standards and specifications. And the voids, if

There are some, are weak points.

  To overcome these difficulties, various solutions

are already known.

  A simple solution is to increase the thickness of the coating over the entire circumference of the tube so as to ensure that the minimum thickness required is respected on the top of the weld seam. This solution obviously consumes more material, so costs more. In addition, this solution does not solve the problem of

empty spaces (bridges).

  Other, more elaborate, solutions have been described (for example by German patents 2,117,618, 2,132,418, 2,260,437). They implement, separately or in combination, two kinds of means: - reduce the tension of the thermoplastic web and slow down the rotation of the tube, at the time of application of the band on the weld. This increases the thickness of material that is deposited in the portion of the circumference of the tube flanking the weld bead (see Figure 6). This also allows the band

  to marry more easily the profile of the weld seam.

  - Deposit on both sides of the weld bead, before applying the coating, a powdered material, or viscous liquid, or prefabricated roll), which

  avoid the formation of voids (see Figure 7).

  These more elaborate solutions however still have drawbacks: the part of the circumference of the tube on which the thickness of material is increased is quite wide: commonly 30 to 40 cm, straddling the weld seam; the tension exerted on the strip between the extruder and the tube. varied. This causes irregularities in the thickness of the coating (thicker on the weld bead, but thinner elsewhere), and / or undesirable residual stresses in some parts of the finished coating; - The implementation is difficult or economically uninteresting for the tubes welded helical; - The filler materials to avoid voids on both sides of the weld seam are heterogeneous with respect to the material of the coating, and can reduce its

adhesion to the tube.

  The object of the present invention is to propose a process which makes it possible to avoid these various disadvantages, as well as

  that a device for implementing this method.

  The principle of the invention consists in compensating for the decrease in thickness of thermoplastic material which will be deposited on the weld bead and in its vicinity on both sides, by the addition of a material supplement

  thermoplastic in the form of sheets in these areas alone.

  According to the invention, the method for coating metal tubes comprising a weld bead in which a plurality of layers of thermoplastic material of a strip wound around a tube is applied to the tube is characterized in that, before or close to application of a first layer of thermoplastic material is applied, straddling the weld bead, sheets of thermoplastic material which are juxtaposed next to each other with a certain overlap edge on edge, so that their juxtaposition covers the weld bead on all its surface as well as a zone of small width framing it on both sides, then one continues applying the (or) layer (s) following (s) of thermoplastic material,

  which one (these) covers these sheets.

  According to this method, as soon as the said bead has been placed on the weld bead, it has been compensated for the decrease in thickness that the prominence of the weld bead causes in the plastic strip which wraps around the tube. The thickness of the sheets is chosen precisely to ensure this compensation; prior experience, or a simple test of coating a tube without these sheets,

allows to determine it.

  The dimensions of the sheets which are suitable for the process are: width (according to the circumference of the tube) suitable for covering the width of the prominent weld bead and a fairly narrow zone, of the order of 30 to 50 mm, of 'other; - length (in the direction of the generatrices of the tube) corresponding to the step -or a multiple of the pitch of the helix according toall the strip of thermoplastic material of the coating wraps around the tube, plus a small complementary length for the recovery from the edge of

  each sheet by the edge of the next.

  The set of leaflets that succeed each other, with the recovery of their edges (in the manner of a deck of cards spread) thus covers continuously the cord of

  welding and a fairly narrow area on both sides.

  The sheets can be - either made and cut separately in advance,

  - either extruded and cut on the spot.

  In any case, they are preheated (to ensure their welding) and applied one after another at the right place and at the right time. t Leaflets may or may not have on their edge (s) one (or more) chamfer (s). The chamfer has the advantage of reducing the stair step effect produced by the overlap of two adjacent sheets edge to edge, which facilitates the proper application of the strip of thermoplastic material that will cover the sheets, and avoids the

void formation.

  It is preferred that the sheet material be the same thermoplastic material as that of the coating tape. Thus the welding of the sheets will be at best,

  and the coating finally obtained will be perfectly homogeneous.

  If the sheets are placed directly on the glue and / or the primary adhesive film, previously applied to the tube, they will be trapped between it and the first layer of the coating. If they are laid after the application of one (or) first layer (s) of coating, they will be trapped between the first of the previously applied layers and the last one of those that will be applied thereafter. Under the effect of the temperature at which the operation takes place, the sheets, made of thermoplastic material, are softened, and are welded to the strip of plastic material which will cover them and, if this is the case, to the strip. of matter

  plastic on which they were applied.

  This gives a coating which has the required thickness regularly on the circumference of the tube, and in particular on the weld bead, and which consists of layers all welded to one another, without voids. As there is no extra thickness at any place, the process is thrifty thermoplastic. It is found that it consumes up to about 20% to 25% less than the methods already known, in the smallest diameters considered. As the rotational speed of the tube, and the tension of the coating strip between its extruder and the tube remain constant, there is no difference in tensions.

residuals in the coating.

  The invention also relates to the device

  intended to implement the method.

  This device is intended to apply to welded metal tubes a coating of thermoplastic material, and preferably comprises: - means for maintaining the tube to be coated, and move it by translation in the direction of its axis, and simultaneously rotate it around of its axis; - Means for winding on the tube an adhesive layer and / or a primary adhesive film, - means for winding on the tube the thermoplastic coating strip. According to the invention, this device is characterized in that it further comprises means for laying astride the weld bead sheets of thermoplastic material, one after the other, s15 each covering a short length of edge of the previous one and then being covered over a short length by the edge of the next, and to apply these sheets to the surface of the weld seam as well as to a weak area.

  width framing it on both sides.

  Other features and advantages of the invention

  will still appear in the description below.

  In the accompanying drawings given by way of non-limiting example: FIG. 1 is a partial sectional view of a welded metal tube; FIG. 2 is a perspective view of a welded metal tube with longitudinal weld; FIG. 3 is a perspective view of a welded metal tube, with helical welding, FIG. 4 represents, in partial section, a coating obtained by the simplest technique, with a reduction in thickness on the weld bead; FIG. 5 represents, in section, such a coating, with moreover voids on either side of the weld seam, FIG. 6 represents, in section, a coating obtained by a technique implementing a reduction the speed of rotation of the tube and / or a reduction in tension of the strip at the passage of the weld seam; FIG. 7 represents, in section, a coating obtained by a technique comprising the addition of a filler material; on both sides of the weld seam - FIGS. 8a, 8b, 8c, 8d show, in section, the steps of the method according to the invention, - FIG. 9 shows the disposition of the sheets on a longitudinally welded tube, - FIG. 10 shows the arrangement of the sheets on a spiral welded tube; - FIG. 11 is a perspective view of a sheet with chamfered edges; FIG. 12 represents, in section, the arrangement of a plurality of successive sheets with their overlaps, FIG. embodiment of the entire device embodying the invention, - Figures 14a, 14b, 14c show in detail the laying means of the sheets forming part of the device

of Figure 13.

  In Figure 1, section of a welded metal tube, we see the metal of the wall of the tube 1, and the cord of

  solder 2, which protrudes outwardly of the tube.

  Figures 2 and 3 show in perspective a welded metal tube, with a longitudinal weld (Figure 2) or helical weld (Figure 3). The wall of the tube 1 and the weld bead 2 are designated by the same references as

in Figure 1.

  Figures 4 to 7 show, in section,

  coatings obtained according to techniques already known.

  In FIG. 4, it can be seen how, with the simplest known technique, in which no particular precaution is taken, the thickness of the coating 3 is lower on the weld bead 2 than on the rest of the

periphery of the tube 1.

  FIG. 5 shows how, under these conditions, can appear, on either side of the weld bead 2, voids 4 between the wall of the tube 1 and the

coating 3.

  FIG. 6 shows a section of the coating 3 that is obtained with a known technique implementing a reduction in the rotational speed of the tube and / or a reduction in the tension of the thermoplastic strip at

passage of the weld seam 2.

  The thickness of the coating 3, which is normally e, increases at the approach of the weld bead 2, to reach E, then decreases to return to e on the top of the weld bead, then increases again to E of the other side of the weld seam, to finally come back to e on the rest of the

periphery of the tube.

  FIG. 7 shows a section of the coating obtained with a known technique involving the application of a filler material on either side of the bead.

welding.

  FIGS. 8a, 8b, 8c, 8d represent, in section, the successive stages of the coating produced according to

  the invention in the area of the weld bead 2.

  In FIG. 8a, the tube 1 is covered only with the first layer 6 of the coating, namely the adhesive and / or the adhesive film, plus, possibly, one (or more) first (s).

  layer (s) of thermoplastic material.

  In Figure 8b, we see a sheet F simply

placed on the weld bead 2.

  FIG. 8c shows this sheet F applied over its entire surface on the layer 6 of the coating covering the weld bead 2 and over a zone of small width of the

  wall of the tube 1 on either side of this weld bead.

  In Figure 8d we see the sheet F covered by

a layer 7 of the coating.

  F sheets of width 1 (according to the circumference of the tube) and length L (in the direction of the generatrices of the tube) are thus successively applied

  all along the weld seam 2.

  FIG. 9 shows how the successive layers Fi, F2, F3, F4,... Are arranged along the weld bead 2 of a longitudinally welded tube 1 of axis AA '. The sheet F2 covers a short length of the edge of the sheet Fi, then will itself be covered for a short length by the edge of the sheet F3, and so on. Thus, all the successive layers completely cover the weld bead 2 as well as a zone of

  width 1 framing it on both sides.

  FIG. 10 similarly shows the disposition of sheets Fi, Fz, F3, F4 on an axis tube AA

with helical welding.

  As will be seen below, the device implementing the method, applies to the weld bead 2, a

  F sheet during rotation of the tube 1.

  The length L of a sheet F must therefore be at least equal to the distance d of which the tube 1 moves in the direction of its axis during one turn of its rotation, plus a small complementary length intended for covering

  on the edge of the previous sheet.

  FIG. 11 is a perspective view of a leaflet

  F rectangular whose edges are chamfered.

  FIG. 12 shows, in normal section at the wall of the tube along the weld seam, how the sheets F1, F2, F3, F4 (represented here with their edges

  chamfered) successively overlap one over the other.

  In Figures 9, 10, 11, the illustrated leaflets have a rectangular shape. This form is not imposed by the process, which would accommodate any other form, provided that it allows, by application of successive sheets with partial overlap, to completely cover the weld bead 2 and a narrow area on both sides. The rectangular shape has the advantage of being obtained very simply by regular cutting of a strip of material

extruded thermoplastic.

  Figures 13, 14a, 14b, 14c show an embodiment of the assembly of a device embodying the invention and clearly illustrate its operation. As shown in FIG. 13, the tube 1, with its weld bead 2 (shown here longitudinally), but which could also be in a helix, is supported by a known member (not shown) which simultaneously ensures its translation in the direction its axis AA in the direction of the arrow T and its rotation about its axis AA in the direction of the arrow R. A side of the tube 1 are mounted the organs

application of the coating.

  A member 10 applies a primary adhesive film 11.

  This member 10 is shown as an extruder from which the adhesive film 11 exits, which is applied and wound around the tube in a helix. The pitch of the helix is equal to the distance d of which the tube 1 moves in translation along the direction T during the duration of one turn of its rotation. A member 20 applies the strip 21 of thermoplastic material of the coating. It is represented in the form of an extruder from which strip 21 is produced which is applied and wound around tube 1 in a helix,

obviously having the same step d.

  It will be appreciated that in Fig. 13 the width of the adhesive film 11 and that of the thermoplastic web 21 are greater than the pitch d of the helix. it

  ensures that the surface of the tube 1 will be completely covered.

  For the adhesive film 11, of which only one thickness is sufficient, a width slightly greater than d is sufficient. For the coating strip 21, whose coating will have several thicknesses, it is possible to choose, as shown, a width

  significantly higher than d or even multiple of d.

  The above is based on known techniques and has been explained only for the proper understanding of the features of the invention, which we will now describe. Between the member 10 applying the adhesive film 11 and the member 20 applying the coating strip 21 is placed an additional member 30 which is characteristic of the invention. This member 30 is fed by a strip 32 of thermoplastic material and, during the rotation of the tube 1, it cuts a sheet F. the laying astride the weld bead 2 and applies it on the surface of the cord and from and other of it. This operation is triggered by an automatism comprising a sensor 31, which detects the passage of the weld bead 2 and then controls the operation of the member 30, with the appropriate delays so that the next sheet is placed just straddling the cord of

  solder 2 when it appears in front of the body 30.

  Leaves Fi, F2, F3, F4 succeed each other, with overlapping edges, as explained above at

9, 10 and 12.

  One embodiment of the organ 30 is

  shown in more detail in FIGS. 14a, 14b, 14c.

  Figures 14a and 14b are sections through a plane perpendicular to the axis AA 'of the tube 1. Figure 14c is a

front view.

  The thermoplastic strip 32 is driven by motorized rollers 33, 33a. It passes into a preheater 34, if it is not extruded in the field, then leaves to stand between the two knives 35a, 35b of a shear controlled by a jack 35. With each rotation of the tube 1, this shears cuts the strip, releasing a sheet F which rests on a holding shelf 36. In Figures 14a and 14c, the two knives 35a, 35b are shown in the open position and the strip 32 passes between them. In FIG. 14b the knives 35a and 35b are shown in the closed position, the sheet F is detached from

band 32.

  As explained above, the width of the fed strip 32, which will be the length L of the sheet F is equal to the pitch of the helix of the movement of the tube 1, plus the small

  length needed to cover the edges of the sheets.

  During the duration of a rotation of the tube 1, the band 32 advances

  the distance that will be the chosen width 1 of the leaflets.

  The sheet F resting on the tablet 36, will, at the appropriate time controlled by the automation, be driven and set up by a pressure roller 37 of flexible material such as silicone rubber, operated by a mechanism & piston 38. The mechanism 38 comprises a cylinder in which is mounted a piston slidable under the action of a fluid. This piston carries a rod 38a whose end carries the roller 37. The mechanism 38 is pivotally mounted along an axis

  Y-Y 'parallel to the tube 1 which is carried by two uprights 38b.

  In FIGS. 14a and 14b, this mechanism 38 is represented in solid lines in its initial position, just before it drives the sheet F. When the automatism is actuated, the piston of the mechanism lowers the pressure roller 37, which drives the edge of the sheet F on the side of the tube 1, applies it to the wall of the tube 1 shortly before the weld bead 2 passes before it, and presses the sheet F over its entire width 1 by matching the profile of the weld bead 2. The mechanism 38 allows the pressure roller 37 to thus accompany the sheet F during its application to the tube 2 which rotates. Figure 14b shows in dashed lines the mechanism 38 with the pressure roller 37 in its final extreme position, where the sheet F, now applied over its entire width 1 on the tube, escapes the pressing roller 37. The mechanism 38 then returns to its initial position (shown in solid lines) ready to operate a

  new cycle with the following leaflet.

  3c5 Of course, the invention is not limited to the embodiment that has just been described and can be made to it many changes without going out

of the scope of the invention.

  Thus the sheets F could be made in a material different from that of the covering strip

wrapped around the tube.

Claims (13)

  Process for the coating of metal tubes comprising a weld bead (2) in which several layers of thermoplastic material are applied to the tube (1) by means of a strip (21) wound around the tube, characterized in that before or after the application of a first layer of thermoplastic material, straps (F1, F2, F3, F4) of thermoplastic material, which are juxtaposed with each other, are applied to the weld bead (2). one beside the other with a certain overlap edge on edge, so that their juxtaposition covers the weld bead (2) over its entire surface and a narrow area framing it on both sides , then we continue to apply the next layer (s) of thermoplastic material, which (s) covers (s) these sheets (Fi, F2, F3, F4).   2. Method according to claim 1, characterized in that the thermoplastic material of the sheets (Fi, F2, F3, F4) is the same as that of the strip (21) wrapped around the tube (1).   3. Method according to one of claims 1 or   2, characterized in that the thermoplastic material of the strip (21) and leaflets (F1, F2, F3, F4) is   polyethylene or polypropylene.   4. Method according to one of claims 1 to   3, characterized in that the leaflets have their edges of chamfered cover.   5. Method according to one of claims 1 to 4, characterized in that the sheets are made on site   by extrusion and cut just before being laid.   6. Method according to one of claims 1 to   4, characterized in that the sheets are prefabricated separately. 7. Device for applying to metal tubes comprising a weld bead (2) a thermoplastic coating, and comprising: means for holding the tube (1) to be coated, and displacing it in translation in the direction of its axis ( A-A '), and simultaneously rotate it about its axis; means (10) for applying to the tube an adhesive layer and / or a primary adhesive film (11); means (20) for winding the thermoplastic coating strip (21) onto the tube; characterized in that the device further comprises means (30) for laying straddling the weld bead (2) one after the other, each covering a short length the edge of the preceding and then being covered on a short length by the edge of the following, and to apply these sheets on the surface of the weld bead as well as on a narrow area of the frame from either side.   8. Device according to claim 7, characterized in that the laying means of the sheets (F, F2, F3, F4) on the weld bead (2) comprise a pressure roller (37) adapted to press each sheet on   Tcute its width by matching the profile of the weld seam.   9. Device according to one of claims 7   or 8, characterized in that it further comprises an organ   (34) preheating the thermoplastic material of the leaflets.   10. Device according to one of claims 7 to 9, characterized in that it further comprises an organ   (35a, 35b) cutting the leaflets.   11. Device according to one of the claims   7 to 10, characterized in that it further comprises a member extruding a strip of thermoplastic material (32) in   which will be cut out the sheets.   Device according to one of the claims   7 to 11, characterized in that the means for laying, pressing, preheating, cutting, extrusion of the sheets (F1, F2, F3, F4) are controlled, with the appropriate delays, by an automatism triggered by a sensor (31) which detects the passage of the weld seam (2) during the rotation of the tube (1   Device according to one of the claims   8 to 12, characterized in that it comprises means for successively performing the following operations: extrusion of a strip of thermoplastic material (32)   - cutting of this strip to obtain a sheet (Fi, F2, Fa, F4) having the desired length, - applying and pressing said sheet on the weld seam (2).